Miscellaneous

Privileged instruction emulation

hwclock and installation hardware detection

inb, outb, sti, cli

Nesting

A milestone

Networking

New network drivers

Checkpointing

Notes:

UML needs privileged instruction emulation for those few processes that try to execute them. These are mostly hwclock running in and out instructions and distribution installation procedures trying to probe hardware. I've also seen daemons using sti and cli.
One interesting application which doesn't currently run under UML is UML itself. It's a demanding process, so being able to run itself will be a sign of maturity and functionality rather than as something that's actually useful.
The virtual ethernet driver needs to be enhanced to allow its daemon to communicate with its peers on other machines, allowing a virtual ethernet to span multiple machines. There are also a couple of upcoming drivers which will need to be integrated.
It's possible to save a process state in a file, and restore it later. In combination with a checkpointing filesystem (which tux2 will do) this will make it possible to checkpoint a UML (including all processes inside it) and restore that state later on.